1. Field of the Invention
The present invention relates to a membrane electrode cell system used in electrocoating, and more particularly to a membrane guard for a membrane electrode cell in an electrocoating paint system.
2. Description of the Prior Art
Electrocoating is broadly classified into two categories, anionic, using anionic paints, and cationic, using cationic paints. Both of these processes are commercially used to deposit paint films on various substrates. As used herein, electrocoating and electrodeposition are considered interchangeable terms.
Membrane electrode cells are commonly used in electrocoating systems and primarily serve two functions. The first function of the membrane electrode cell is to act as the opposing electrode in the electrocoating process, with the object being painted serving as the counter-electrode. The second function is to serve much like a dialysis cell or electro-chemical cell in which ions are removed from the paint bath to maintain proper paint bath chemistry. The membrane electrode cell can have many shapes, and often is shaped as a flat rectangle, semi-circle, tube or cylinder. An electrocoating process employing such a membrane electrode cell is disclosed in U.S. Pat. Nos. 4,851,102, 4,711,709 and 4,834,861, which are hereby incorporated by reference.
The membrane used in a membrane electrode cell can be either ion-exchange or neutral. It often is comprised of a composite of resin, binder, and flexible substrate, and typically is rather fragile and susceptible to damage. An example of an anion-selective membrane is Model Number MA3475, manufactured by Sybron Chemical, Inc. An example of a cation-selective membrane is Model Number MC3470, also manufactured by Sybron Chemical, Inc. An example of a neutral membrane is Cellgard Model Number 5511, manufactured by the Celanese Corporation.
The membrane is arranged in such a fashion as to separate the electrocoating paint bath from the electrode. An electrolyte fluid flows between the inside of the membrane and the outside of the electrode. This electrolyte fluid, which is comprised mostly of deionized water and a small amount of acid or amine (depending on the type of electrocoating employed), is responsible for flushing the ions that pass through the membrane into the membrane electrode cell from the paint bath. The conductivity of this electrolyte fluid usually is maintained in the range of 500 to 2,000 microSiemens/cm (microMho/cm).
The flow of electricity from the electrode to the counter-electrode must pass through the electrolyte fluid, membrane, electrocoating paint bath and eventually the deposited paint film. If the resistance of any of these elements increases, then the driving voltage generally must also be increased to maintain the same flow of current. The thickness of the deposited paint film (typically 0.5 to 1.5 mils) is directly related to the number of coulombs (ampere/seconds) that pass between the electrode and counter-electrode. Therefore, any reduction of the flow of electrical current results in a reduction in the rate of paint film deposition. Typical driving voltages are between 150 and 350 volts. Once the voltage goes higher than a certain level, paint film defects can occur from "rupture", where tiny air bubbles trapped in the film cause a rough film appearance.
A recurrent problem for most electrocoating systems is the loss of counter-electrodes from the conveyor hooks that move them in and through the paint bath. Some electrocoating systems paint a wide variety of sizes and shapes of counter-electrodes. Often the hooks either are not optimally designed for each and every different counter-electrode, or the counter-electrodes are incorrectly hung on the hooks. In any event, as the counter-electrodes enter the paint bath, the buoyant forces caused by the immersion of the counter-electrode into the paint bath sometimes lift them off of the hooks, and the release of trapped air from inside the counter-electrode can cause wild swings back and forth. The combination of these movements with the conveyor motion and/or paint bath agitation often causes the loosened counter-electrode to fall completely off the hook. In some cases, a trailing counter-electrode that is on a hook directly behind the loose counter-electrode can also become entangled and cause successive counter-electrodes to pile up, much like an automobile chain collision.
As counter-electrodes come loose and fall off their hooks, they can come into physical contact with the membrane electrode cells that generally are arrayed along the long sides of the paint bath tank. Also, maintenance personnel sometimes use long-handled grappling hooks to remove the fallen counter-electrodes and either the grappling hook or the retrieved counter-electrode can come into contact with the membrane electrode cell. Since the membrane can represent up to about 90 to 95% of the exposed surface area of the membrane electrode cell, it is especially vulnerable to physical damage if any object comes into direct physical contact with the membrane electrode cell.
If the membrane suffers a cut, puncture, hole or rip, then its functionality can be severely and adversely affected. Once an opening, or "short-circuit" path, through the membrane is created, the membrane no longer can effectively remove ions or easily allow the passage of current to the counter-electrode, thereby impeding or stopping altogether the electrocoating process. Two things occur almost immediately after a membrane is penetrated by a counter-electrode or other object. The first is that the electrolyte fluid becomes contaminated with paint. Since the paint particles carry the same charge as the electrode, they are repelled by the electrode. With no ready way out, these paint particles attempt to escape through the membrane. This often results in the deposition of the paint particles on the inner surface of the membrane because they are too large to migrate through the small passages of the membrane. This phenomena "fouls" the membrane, and the resistance of the membrane can dramatically increase.
The second problem occurs thereafter. With the membrane fouled, it no longer effectively removes ions from the paint bath. With the ion-removal process disrupted, the chemical balance of the paint tank is soon upset.
Over the years many attempts have been made to decrease the incidence of damaged or compromised membranes. Polyvinyl chloride (or PVC) pipes, sometimes called rub rails, have been positioned between the membrane electrode cell and the counter-electrode. If a counter-electrode swings from side-to side, then these rub rails tend to keep the counter-electrode from contacting the membrane electrode cell. Normally, two or three rubs rails are equally spaced vertically between the top and the bottom of the counter-electrode. While rub rails do offer some degree of protection, the size and shape of counter-electrodes vary to a high degree, limiting the effectiveness of this approach. Moreover, it is not practical to put rub rails throughout the paint bath, since this would block the free-flow communication of paint and electricity between the electrodes and also physically reduce the working volume of the paint bath. A further disadvantage of rub rails is that membranes can still be damaged if corners of sharp edges of certain counter-electrodes pass in between or around a rub rail and collide with the membrane.
Another method of preventing physical contact with the membrane electrode cell employs non-conductive, perforated barriers (material such as PVC or fiberglass) that may be as much as 1 inch deep with 1 inc by 1 inch openings. This method overcomes some of the problems associated with the PVC rub rails in that the network of openings in the barrier can be smaller then the gaps between the PVC rub rails. A disadvantage of this method, however, is that the depth and relatively large exposed surface area of the barrier create a significant disruption to the free-flow communication and circulation of the paint.
A high percentage of the electrocoat paint bath is water. The remainder mostly is paint resin, pigment, neutralizer and solvent. The paint bath must be vigorously agitated on a substantially continuous basis or the paint particles will tend to fall out of solution and gather at the bottom of the tank. Hence, any object inside the tank that presents itself as a significant flow or circulation restrictor, especially one with flat, horizontal or vertical surfaces, will tend to cause paint particles to fall out of solution and also disrupt the even and orderly lines of current between the electrode and the counter-electrode. If these paint particles begin to coagulate, they can start to gather and pile up on any flat ledges or openings of any protective barrier. If left unchecked, the coagulated particles can "grow" to a level where small, semihardened pieces can flake off. These coagulated paint particles can then settle out on the counter-electrodes and cause a myriad of paint film defects. Once paint particles begin to coagulate, the task of getting those paint particles back into solution can be a costly and time consuming undertaking. In addition, the driving voltage must be higher than otherwise to overcome the restrictions to the flow of electrical current caused by this kind of protective barrier, which is less efficient in terms of energy consumption and paint bath cooling.
Large diameter PVC pipes with numerous holes (typically 1/2 to 3/4 inches) drilled in them have also been used in an effort to protect the membrane. The membrane electrode cell can be placed inside of these pipes, offering some degree of protection. This method suffers, however, from the same drawbacks of the external barriers. The wall thickness of the large diameter (say 4 or 6 inches) pipes can be as much as 0.432 inches. Once a hole is drilled through the pipe and it is then placed in a vertical position, the bottom side of the hole acts as a ledge. As the undrilled portions of the pipe act as a flow restrictor to the free-flow of paint, small amounts of paints particles fall out of solution and settle on this ledge. The undrilled portions of the pipe also act to disrupt the free-flow communication of the current much like other prior art barriers.
Yet another structure has been fabricated from large flat sheets made of non-conductive material (such as PVC or nylon). One prior art structure covered a planar electrode with a flat steel grill coated with a plastic material. Many holes (say 1/4 to 3/4 inches) or other perforations are drilled or cast into the flat sheet or grill. This sheet may be as thick as 1/2 inch. These structures, like the drilled PVC pipe, offer some protection for the membrane, but they also suffer form the same disadvantages discussed above. Moreover, the plastic coated steel grill structure suffers from several additional drawbacks. The plastic coating can be scratched or nicked, providing a site for paint deposition. Because the paint typically does not cure, it eventually falls off, resulting in paint film defects. Further, the conductivity of the steel in the grill poses undesirable electrical isolation problems.
Another method for protecting the membrane electrode cell involves the use of a non-conducting mesh material wrapped around the membrane. This mesh has much smaller openings than discussed above and is used in such a way that the mesh makes direct contact with the membrane. While the mesh does offer some protection for the membrane, it has several drawbacks. For example, since the mesh makes direct contact with the membrane, some of the membrane passages are completely blocked off, thereby reducing the efficiency of the membrane. In addition, the mesh can inadvertently chafe or abrade the membrane at the contact points, resulting in damage to the membrane. Moreover, paint particles can build up on the horizontal surfaces of the mesh where the mesh makes contact with the membrane.
Another version of this approach uses long slender rectangular-cross section, non-conducting pieces with small inter-connecting supports arrayed to form a grill completely over the membrane. Since the supports used in this approach also come in direct contact with the membrane, it suffers from the same disadvantages. Another drawback to this approach is that, since the grill is mostly one-directional, in some instances it can act as a guide or channelling mechanism and direct counter-electrodes or other objects into contact with the membrane.
Even though these prior methods more or less offer some protection for the membrane electrode cell, they all create drawbacks that ultimately can be just as series as the damage caused when a membrane is penetrated. Hence, it is desirous to develop a guard that protects the membrane, but does not disrupt the free-flow communication of paint particles or electrical current vital to a properly-functioning electrocoating system.